Europe’s new Mars mission will take aim at one of planetary science’s most intriguing mysteries.

On Monday evening (European time), the first mission of Europe’s two-part ExoMars program was successfully launched on a Russian Proton rocket. The spacecraft is now on the way to Mars, where it’s scheduled to arrive in October. Actually, there are two spacecraft: the Trace Gas Orbiter and a landing module called Schiaparelli, which will separate three days before arrival. Schiaparelli is scheduled to enter the atmosphere and land on Mars (a first for Europe) on October 19. Science operations will begin the same day, while the TGO’s science mission won’t begin until December 2017, when it reaches its final orbit 400 kilometers above the Martian surface.

Schiaparelli’s main purpose is to demonstrate that the European Space Agency (ESA) can execute a soft landing on Mars, as it plans to do on the next ExoMars mission, a rover due to launch in 2018 or 2020. The test lander’s scientific objectives are rather modest. Once it lands on the surface, Schiaparelli will measure the air temperature and pressure, humidity, wind speed and direction, and the transparency of the atmosphere.

ExoMars’ Trace Gas Orbiter is more ambitious scientifically. It will measure the composition of the Martian atmosphere, particularly biologically relevant gases such as methane and its degradation products. The orbiter also will try to pinpoint the locations and sources of these gases. If methane emissions are coming from volcanic areas, it might indicate a geochemical (non-biological) origin. Discovering that they’re coming from active hydrothermal areas instead would be exciting, because it would suggest that we have found potentially habitable oases in the Martian frigid desert, with heat and water necessary to support life. Most exciting of all would be an association with one of the so-called Special Regions on Mars, locations believed to have a high probability of supporting current Martian life, some of which could be giving off methane.

We’ve learned from NASA’s Curiosity rover that sniffing out the methane will be challenging, however. Concentrations of the gas are extremely low at Gale Crater—in the low parts-per-billion range—and are likely to be even lower for methane’s degradation products. The expectation is that concentrations will be much higher near the sources than anywhere else on the planet. If so, TGO may point us to a suitable landing site for the second ExoMars mission. That rover will have a drill, and its objective will be to hunt for biosignatures of past, or possibly even present, life.